Ughhh what do I do! How do I say no?

why dont you tell josh just what you told us? ask him if he would be willing to go with you and her to the fair. You said you and him will probably just sit at home anyways so he would probably be ok with it. tell him you dont know how to bow out graciously, but if he came along you wouldnt need to. If he cant go or doesnt want to, maybe you can tell your friend that you will go with her but put a shorter time limit on the night. like tell her you can only stay 2 hours,...or you could "become ill" after you get there and tell her you need to go home.